FIG. 1 shows an example of a conventional system. The example shown in FIG. 1 switches from the current line to the protection line by switching an electrical signal before the multiplexing.
At a transmitting (master) side, 45 MHz signals amounting to 72 channels are input to a switching unit 11. Out of these signals, the electrical signals amounting to 36 channels are multiplexed and converted into an optical signal of 1.8 GHz per line.
The multiplexing of the electrical signals and the conversion to the optical signal are carried out in a protection side transmission unit 12 and a current side transmission unit 13. In FIG. 1, only the current side transmission unit 13 out of two current side transmission units is shown.
Optical transmission lines OF1 and OF2 respectively transmit the optical signals from the protection side transmission unit 12 and the current side transmission unit 13 to a receiving (slave) side.
The receiving (slave) side includes a protection side transmission unit 15, a current side transmission unit 16 and a switching unit 17 in correspondence with the transmitting (master) side. Normally, a communication is made via a current line which is formed by the current side transmission units 13 and 16 of the respective transmitting and receiving sides and the current optical transmission line OF2 which is connected to these current side transmission units 13 and 16.
If a fault occurs in the current line, a switching is made from the current line to a protection line by control operations of a switching controller 10 on the transmitting (master) side and a switching controller 14 on the receiving (slave) side.
That is, at the transmitting (master) side, the electrical signals which amount to 36 channels and are connected from the switching unit 11 to the current side transmission unit 13 are switched one channel at a time and successively connected to the protection side transmission unit 12. Similarly, at the receiving (slave) side, the electrical signals which amount to 36 channels and are connected to the current side transmission unit 16 are switched one channel at a time and successively connected to the protection side transmission unit 15. Accordingly, the optical signal which is transmitted from the protection side transmission unit 12 via the protection transmission line OF1 is converted into electrical signals in the protection side transmission unit 15, and output via the switching unit 17.
Hence, in the case of the conventional system shown in FIG. 1, the switching between the current and protection sides is made in a state before the electrical signals are multiplexed. The switching is made one channel at a time at each of the switching units 11 and 17, and the application of this system to a high-speed synchronized network is difficult because it takes considerable time to carry out the switching.
Accordingly, there is a proposal to carry out the switching of the lines in a state where the signal is already a multiplexed optical signal, as shown in FIG. 2. In FIG. 2, the protection side transmission unit 12 on the transmitting side is coupled to an optical transmission line OF3, an optical coupler (OC) 18, the current side transmission unit 13, and a plurality of current side transmission units of the subsequent stations which are not shown. Similarly, the protection side transmission unit 15 on the receiving side is coupled to the current side transmission unit 16 and a plurality of current side transmission units of the subsequent stations which are not shown, via an optical transmission line OF4 and an optical coupler 19. Thus, a plurality of current lines exist with respect to one protection line.
If a fault occurs in one current line, an optical signal of 1.8 GHz or the like and multiplexed by the current side transmission unit is guided to the protection side transmission unit 12 via the optical transmission line OF3 under the control of the transmitting (master) side switching controller 10, and is sent to the receiving (slave) side via the protection optical transmission line OF1.
At the receiving (slave) side, the optical signal sent via the protection side optical line OF1 is received via the protection side transmission unit 45 under the control of the switching controller 44.
In the case of the example shown in FIG. 2, an optical receiver (not shown) for receiving an incoming optical signal from the optical transmission line OF3 of the protection side transmission unit 12, suddenly receives the switched optical signal from a state where no signal is received. In this case, it takes approximately 100 msec until the received light stabilizes and an optical signal which is pulled into synchronism is guided to the protection optical transmission line OF1.
On the other hand, according to the SONET standard which is set for the optical transmission systems of synchronized networks employed in the United States and the like, the switching time to the protection line is prescribed as 50 msec or less. Hence, even if the optical signal can be switched directly, there was a problem in that it is difficult to switch the line within the time prescribed under the SONET standard.
Accordingly, it is an object of the present invention to provide a novel and useful optical transmission system in which the above described problem is eliminated.
In addition, it is also an object of the present invention to provide an optical transmission system which can reduce the current/protection switching time and satisfy the switching time prescribed under the SONET standard.
Further, it is an object of the present invention to provide an optical transmission system which can detect the line quality between a protection side transmission unit and each of a plurality of current side transmission units.